In an industrial robot or the like equipped with a multi-joint arm, an electric motor and sensors provided in each of the articulated joints corresponding to the shoulder, elbow and wrist are required to be fed with power and control signals from a control circuit incorporated in the main body of the robot. The cables for conducting the power to the electric motor and the control signals to the sensors extend along the length of the arm, and are typically given with a slack at each of the articulated joints to accommodate for the flexing movement of the articulated joint.
As methods for passing a cable through an articulated joint, it is known to provide a cable passage that bypasses the bearing portion of the articulated joint (as disclosed in Japanese patent laid open publication No. 63-288692) and to use a hollow shaft in the articulated joint to pass the cable therethrough (as disclosed in Japanese patent laid open publication No. 61-168478).
However, according to the technique disclosed in Japanese patent laid open publication No. 63-288692, because the wiring duct for protecting the cable is provided on the exterior of the articulated joint and there is no difference in that the cable extends along the exterior of the articulated joint, an increase in the external dimensions of the articulated joint cannot be avoided.
However, according to the technique disclosed in Japanese patent laid open publication No. 61-168478, because the cable is passed through the inner bore of the hollow shaft, a length of the cable corresponding to the length of the shaft must be spent at the articulated joint so that the total length of the cable becomes relatively large for the given length of the arm and this prevents a compact design of the articulated joint.